[57.05] Callisto during the Galileo Europa Mission: Geology and stratigraphy of the C20 and C21 target areas

Callisto was imaged by the SSI camera onboard the Galileo
spacecraft in three close flybys during the Galileo Nominal
Mission, and recently in two close flybys (C20, C21) during
the Galileo Europa Mission. These images were geologically
mapped, and crater size-frequency distributions were
measured to define the sequence of geologic events. Absolute
ages were obtained from two cratering chronology models: one
based on a lunar-like cratering history, preferentially from
asteroids (Neukum and others) (Model I), another one based
on a constant cratering rate from Jupiter-family comets
(Zahnle and others) (Model II). An area characterized by a
dark ''flow'' in low-resolution Voyager images was targeted
in C20. Despite this feature was conjectured to have been
created by cryovolcanism, the higher-resolution SSI frames
(100 and 430 m/pxl) do not reveal past endogenic activity.
Instead, there is no distinct boundary between the flow-like
patch and adjacent dark material. Dark, blanketing material
which has been shown to be globally abundant on Callisto
embays bright material of a heavily degraded impact
structure (~150 km) not revealed on Voyager images.
Measuring the size-frequency of craters superimposed on the
degraded feature shows that it may have formed between the
events which created the Asgard and Valhalla multi-ring
basins, between about 4.2 billion years (b.y.) and 4 b.y.
ago (Model I), or between about 4.3 b.y. and 2.3 b.y. ago
(Model II). Also in C20, Bran, a large crater featuring
bright rays was targeted. The rim seen in Voyager data
appears to be an outer ring, the actual crater rim is
smaller in diameter (~70 km). Based on crater
size-frequency measurements on the continuous ejecta, this
crater was formed about 4 b.y. ago (Model I), or 2.45 b.y.
ago (Model II).